The invention concerns a thermal power station with a gas turbine having connected after it a heat exchanger in which part of the heat still contained in the exhaust gases of the gas turbine is used to heat a plant for seawater desalination.
Steam turbines are usually employed to operate plants for desalinating seawater, the exhaust steam from the turbines serving as a heat source for the distillation process in that the condenser heats the seawater used as a coolant. (Publication of the Societe Internationale de Dessalement (SIDEM), Paris: Production d'eau douce par dessalement.)
The most economical temperatures for the distillation process lie between about 150.degree. and 180.degree. C, which corresponds to a back-pressure at the steam turbine of some 5 to 6 bar. In the case of very expensive fuels, relatively high plant costs are still economical, and one can resort to a back-pressure of some 2 to 3 bar; this corresponds to a steam temperature of about 120.degree. to 130.degree. C.
It is often more efficient to use a gas turbine instead of a steam turbine as the heat source for the desalination plant, the exhaust gases of the gas turbine being utilized in a heat exchanger to heat high-temperature hot water for the seawater desalination process (Brown Boveri Rev. vol. 54 (1967) p. 9-16). Unfortunately, the temperature of the exhaust gases is very high; in present-day gas turbines it is normally 450.degree. - 550.degree. C. Lowering this temperature by raising the pressure ratio through the gas turbine would seriously impair the thermal process because output and efficiency of a gas turbine fall sharply when the optimum pressure ratio is exceeded.
The exhaust gas temperature is therefore too high for the desalination plant because it results in unacceptably high steam temperatures in the distillation, and hence leads necessarily to unacceptably high steam pressures. The high steam pressures, however, would make the multistage cascade evaporation process very expensive, or indeed impracticable. Furthermore, the high temperatures would cause fouling of the tubes due to the salt contained in the seawater. But if the temperature of the high-temperature hot water for the distillation process is reduced to about 120.degree.-170.degree. C, the valuable heat at the outlet from the gas turbine is severely degraded owing to increased entropy.
The seawater desalination plants in use today are therefore unsatisfactory from the economic standpoint.